Interstellar medium Flashcards
What are the general properties of the interstellar medium?
It occupies the space between stars in spiral and irregular galaxies. It is at about 10^6m-^3. However it exists in many temperatures and densities.
Where does ionized gas exist?
Near hot stars, high up on the main sequence, where the photons ionize the atoms. UV is needed.
Where do Hll regions exist?
Hot young massive stars that are still surrounded by dense 10^10m_^3 gas that becomes ionized
How are Hll regions viewed?
The gas is hot so fluoresces at around 300nm.
They are also viewed through an emission line spectrum of the atomic hydrogen. Also when the electrons recombine with the hydrogen ions, energy is released.
When the electrons are accelerated towards the protons radiowaves are emitted
Where are HI regions found?
They are typical densities, where it is around 100K
How are HI regions viewed?
They do not have enough energy to be above the ground state. However there is a hyperfine transition in the ground state between spin up and spin down releases a photon of 21cm.
Where is molecular gas found?
Cold dense regions
How is molecular gas viewed?
Hydrogen molecules are symmetric so do not emit. Carbon monoxide is used instead. It emits when rotational transitions occur due to collisions. The ground state transition is at 3mm
How is the presence of dust noted?
Dust lanes block out background starlight.
What is total extinction?
The amount of dimming measured in magnitudes
Why does dust redden the background objects?
The dust absorbs more blue light than red light because the size of the dust grains is similar to the wavelength of blue light.
What is colour excess?
The amount of reddening measured in magnitudes. It is the difference between the observed and intrinsic colour.
What is the extinction law?
Total extinction = 3 colour excess
What are near - infrared wavelengths used to view background objects?
Extinction drops as wavelength increases
What wavelengths do dust grains emit at?
In interstellar space they emit at far infrared, when temperatures are 30K. When they are near a hot star they heat up and emit at mid infrared.
